There have been already provided such sensors of the theft and the like of automobiles as a sensor 1 utilizing the resonance of a vibrating plate of a hat-like cross-section as shown in FIG. 4, a sensor 2 utilizing the resonance of a disk-like vibrating plate fixed on the outer periphery as shown in FIG. 5 and a sensor 3 utilizing an electromagnetic induction as shown in FIG. 6.
In FIG. 4 showing the sensor 1, the reference numeral 4 represents a housing, 5 represents such rigid vibrating plate as a metal plate of a hat-like cross-section with both feet fixed to the housing 4, 6 represents a piezo-electric element pasted to the vibrating plate 5 and 7 and 8 represent lead wires for taking out the output signals of the piezo-electric element 6. This sensor 1 is fitted in such proper space as within the passenger compartment of an automobile so that the vibrating plate 5 may vibrate with the rocking of the automobile at the time of the theft, the vibration may be converted to an electric signal by the piezo-electric element 6, the signal may be output from between the lead wires 7 and 8 and the theft and the like may be thereby sensed.
However, in this sensor 1, as the above mentioned vibrating plate 5 is used, there has been a defect that the resonant sharpness is so large that, in case specific conditions are not met at the time of the theft, for example, in case the kind and loaded weight of the automobile are different, the theft and the like of the automobile will not be able to be positively sensed. As the rocking of the car body by the loaded body weight of the intruder into the passenger compartment vibrates at such low frequency as generally mostly below several 10 H.sub.z, there has been also a defect that, in order to sense the vibration of this low frequency, the above mentioned sensor 1 must be formed to be large in the structure of the vibrating plate 5 and can not be made small.
In FIG. 5 showing the sensor 2, the reference numeral 9 represents a housing, 10 represents such rigid vibrating plate as a disk-like metal plate fixed on the outer periphery to the housing 9, 11 represents a piezo-electric element pasted to this vibrating plate 10 and 12 and 13 represent lead wires for taking out the output signals of the piezo-electric element 11. The same as the above mentioned sensor 1, this sensor 2 is fitted in such proper space as within the passenger compartment of an automobile so that the vibrating plate may vibrate with the rocking of the automobile at the time of the theft, this vibration may be converted to an electric signal by the piezo-electric element 11, this signal may be output from between the lead wires 7 and 8 and the theft or the like of the automobile may be thereby sensed.
However, even in this sensor 2, the same as in the above mentioned sensor 1, there has been a defect that, in order to sense a low frequency vibration, the sensor 2 must be formed to be large in the structure of the vibrating plate 10 and can not be made small.
Further, in FIG. 6 showing the sensor 3, the reference numeral 14 represents a housing, 15 represents an elastic string provided as stretched in this housing 14 and 16 represents a weight provided in the middle of this elastic string and formed of a permanent magnet 17 exposed on the lower surface. Also, a copper wire coil 18 is provided below the above mentioned weight 16. This sensor 3 is fitted in a proper space of an automobile. The principle of the vibration of this sensor 3 is of a torsion bar system with the elastic string 15 as an axis so that, when the automobile is vibrated, the weight 16 will swing as a pendulum with the elastic string 15 as an axis, the magnetic flux intersecting the copper wire coil 18 by the permanent magnet 17 of the weight 16 will vary, an induction current will be generated in this copper wire coil 18 and the theft and the like of the automobile will be thereby sensed.
However, in the above mentioned sensor 3, there has been a defect that, when the plane in which the above mentioned copper wire coil 18 is arranged inclines much as in the case that the car body inclines much while parked, the weight 16 will rotate to be directed in the perpendicular direction by the gravity, the permanent magnet 17 of the weight 16 will separate away from the copper wire coil 18 and the sensitivity will be thereby reduced. There has been also a defect that, in case the car is parked above high voltage lines embodded under the ground or in case there is in the vicinity a transmitting antenna generating strong electromagnetic waves, under the influence of the electromagnetic induction from outside, an induction current will be generated in the above mentioned copper wire coil 18 and a mis-operation or operation failure by an excess input will be caused.
Also, as a sensor of the theft and the like of an automobile, there has been provided a sensor provided with a vibration converting part for converting the vibration of the car body to an electric signal of a frequency coinciding with the frequency of this vibration over a wide frequency band without utilizing the resonance.
However, in this sensor, a filter has been indispensable to prevent a mis-operation from being caused by an external acoustic noise or meteorological factor. That is to say, as the above mentioned vibration converting part converts to an electric signal not only the vibration of the car body of a low frequency generated at the time of the theft or the like but also the vibration of the car body of a comparatively high frequency based on such external sound as of a klaxson of another car, siren of an emergency car, rainfall or wind or on a meterological factor, a filter for taking only the signal based on the vibration of the car body of a low frequency generated at the time of the theft out of the electric signals has been indispensable to the above mentioned sensor.